Current automobiles are demanded for high efficiency and a clean-up of exhaust in view of environmental protection and energy conservation.
Means for achieving high efficiency includes an improvement in an engine structure by downsizing, a reduction in the number of cylinders, an increase in compression ratio, etc. The downsizing is implemented by a geometrical structure of an engine and thus cannot be changed upon engine operation. The reduction in the number of cylinders and the increase in compression ratio can be changed upon engine operation. The reduction in the number of cylinders can be simulated by the pausing of a cylinder. The compression ratio is calculated with reference to the position of a piston or is calculated with reference to the timing at which an intake valve is closed (i.e., the timing at which a mixture starts to be compressed). In the case of the calculation with reference to the piston position, the compression ratio is calculated by a ratio between a combustion chamber volume for when the piston is at top dead center and a volume in a combustion chamber for when the piston is at bottom dead center. In the case of the calculation with reference to the intake valve closing timing, the compression ratio is calculated by a ratio between a volume in the combustion chamber at the time when the intake valve is closed and a combustion chamber volume for when the piston is at top dead center. Thus, the compression ratio can be made variable by the piston stroke or the intake valve closing timing.
Means for achieving a clean-up of exhaust includes early activation of a catalyst provided in an exhaust manifold. Specifically, a catalyst warm-up mode that increases exhaust temperature by retarding ignition timing after the engine starts is performed. In relation to control to retard the ignition timing, a slightly rich mixture is placed around an ignition plug in order to stabilize combustion. However, there is a possibility of an increase in particulate matter/number (hereinafter, PM/PN) due to a reduction in the temperature in the combustion chamber caused by the presence of the slightly rich mixture and the retard of the ignition timing. Specifically, there is a possibility that fuel that is oxidized at low temperatures may become soot, producing PM/PN.
The following PTL 1 discloses a technique read as “By allowing the valve lift characteristics of a variable valve mechanism upon secondary-air supply to have a predetermined secondary-air lift setting, unburned gas including hydrocarbons (HC), etc., which are emitted to an exhaust passage without being burned in a combustion chamber of an internal-combustion engine can be increased without an air-fuel ratio (combustion A/F) in the combustion chamber becoming significantly rich which is caused by an increase in the amount of fuel. That is, both the suppression of a rich combustion A/F (the achievement of a lean combustion A/F) and the increase in the concentration of HC emitted from the combustion chamber can be achieved. Therefore, by the suppression of a rich combustion A/F, an improvement in fuel efficiency and a reduction in exhaust emissions such as NOx are achieved, and by the increase in the concentration of HC, due to the promotion of afterburning by secondary-air supply, an effect of increasing exhaust temperature increases, enabling to achieve early catalyst activation.” (see paragraph 0007).